The subject invention relates to transmission systems in which an endless, flexible transmission medium (e.g., a chain, a belt, etc.) is driven at constant speed by a rotating driving member. The transmission medium drives the rotatable driven member. A pair of idler elements on opposite sides of the driven member and driven by the transmission medium are rotatably mounted on a linkage means (e.g., a rockable arm). The linkage means is moved in periodic, cyclic, linearly reciprocal or oscillating motion by which there is superimposed on the constant speed drive of the driven member by the transmission medium a secondary velocity which cyclically varies the speed of rotation of the driven member.
The invention also involves dual wire-mesh belt compaction modules for the wetout of glass fiber rovings and sheet molding compound paste between synthetic films sandwiched between the belts, wherein the subject transmission systems are employed to drive one of the drum rolls over which the one wire-mesh belt passes at cyclically-varying linear speed. The cyclic changing of the speed of one wire-mesh belt vis-a-vis the other, constant speed belt aids in attaining a more uniform wetout of the glass fiber rovings by the sheet molding compound by the shearing action derived from cyclic variations of the relative speeds of the upper wire-mesh belt and the lower wire-mesh belt.